1. Field of the Invention
The present invention relates to an apparatus for detecting ferromagnetic objects and in particular to a device for detecting the presence of ferromagnetic objects in the vicinity of magnetic resonance imaging (MRI) scanners.
2. Discussion of Prior Art
Most major hospitals have rooms for Magnetic Resonance Imaging (MRI) scanners. These scanners have a large magnet that is usually superconducting and produces a very high magnetic field up to several Tesla within the bore of the MRI scanner. The magnetic field strength outside of the magnet falls rapidly with distance creating very large magnetic field gradients in the surrounding room. Any ferromagnetic metal object in the vicinity of the magnet will experience a force attracting it towards the magnet. The force exerted by the magnet may be sufficiently strong to accelerate an unrestrained ferromagnetic object towards the MRI scanner, where it will come to rest in or near to the bore of the MRI scanner. This is called the projectile effect or missile effect and it can be very dangerous and damaging.
Large ferromagnetic metal objects undergoing the projectile effect can enter the bore of the MRI scanner with sufficient kinetic energy to injure a patient or damage the MRI machine extensively. Furthermore, such objects may be impossible to remove from the bore without switching the magnetic field off. It can take over a week to restore the field and the down-time can be expensive for the hospital.
Examples of problem ferromagnetic objects that cause projectile effect accidents include gas bottles (small and large), wheelchairs, tool boxes, mop buckets, vacuum cleaners, pens, scissors and various medical devices, for example defibrillators and respirators.
Because of these dangers a strict screening procedure is enforced that is usually adequate in ensuring that staff and patients are free of ferromagnetic metal objects before entering the room in which the MRI scanner is located. However, there are a few major instances of projectile effects in the world every year and many minor incidents. Each major incident is usually very costly to the hospital or their insurers.
Metals that are non-ferrous do not present this danger and are used routinely in MRI rooms. Metal items for use in MRI rooms are usually pre-approved. However, it is often difficult for people to know if a metal is ferrous or non-ferrous and it is not always convenient to check for approved items. Accordingly, there is always a danger of the projectile effect due to oversights and mistakes on part of staff and patients, and general human error.
Installing metal detectors at the entrance to hospital rooms in which MRI scanners are located might help reduce the incidence of MRI related accidents (The New England Journal of Medicine 2001; 345; pp 1000-1001). For example, it has been suggested that an archway metal detector, similar to those employed at airports, could be placed at the entrance to an MRI room to detect metal objects which might pose a danger.
However, there are several difficulties with the above suggestion which have hitherto precluded the use of conventional metal detectors for screening persons in the vicinity of an MRI scanner.
Firstly, the metal detector would have to reliably discriminate between ferromagnetic and non-ferromagnetic metals otherwise it would alarm on approved metal objects. Not all conventional archway metal detectors are capable of such discrimination.
Discriminating metal detectors are available, however such devices tend to transmit relatively large amounts of electromagnetic energy. This is not desirable in a clinical environment where sensitive equipment abounds.
Moreover, conventional archway metal detectors are primarily aimed at security applications rather than safety applications and typically exhibit a high degree of sophistication (see for example U.S. Pat. No. 3,971,983). Consequently, sophisticated archway metal detectors are prohibitively expensive for use in MRI screening applications.
Furthermore, conventional archway metal detectors are physically incompatible with the beds, trolleys and wheelchairs used in a hospital environment (see for example U.S. Pat. No. 6,133,829).
Finally, conventional metal detection systems aimed at security applications are almost exclusively attended by an operator who will take appropriate action in response to a visual or audible signal from the metal detection system. In contrast, a screening device for an MRI scanner must operate automatically to provide an audible/visual warning of a potential danger, and even prohibit access to the MRI scanner if appropriate.
It is an object of the present invention to mitigate at least some of the disadvantages of the foregoing metal detection systems. It is a further object of the present invention to provide an alternative device for detecting ferromagnetic objects in the vicinity of an MRI scanner.